From 85 occupational cohorts of workers exposed to organic solvents, we have previously found a low weighted standardized mortality ratio (SMR) for all causes (SMR=89.5,95 per cent confidence interval [CI]=89.0-90.2).

Characteristics of the cohorts were examined for association with the HWE.

A multiple regression analysis revealed that, for overall deaths, the closer the comparison group, the smaller the HWE (p=0.001) ; the more hazardous the potential exposures, the higher the SMR (p=0.02) ; the higher the social class, the greater the HWE (p=0.02) ; and the higher the rates of loss to follow up, the greater the HWE (p=0.04).

These results have shown that, in addition to time-related modifiers, the comparison group, type of occupational exposure, social class and rates of loss to follow-up significantly influence the size of the so-called HWE.

Differences in the HWE related to gender and race were probably attributable to different rates of loss to follow-up.